The present invention relates to a fuel injection control system and a control method for a two-cycle in-cylinder direct injection engine.
As a fuel injection control method for a two-cycle engine in which fuel is injected directly from an injector into a combustion chamber, there has been proposed a method in which fuel injection timing is controlled so that at the time of low-speed rotation of an engine, at least 80% of the fuel to be injected is injected on a compression stroke after an exhaust port has been closed, as described in U.S. Pat. No. 4,807,572.
As publicly known, the quantity of fuel injected from the injector is determined by multiplying a differential pressure between the pressure of fuel applied to the injector (fuel pressure) and the pressure of a space into which fuel is to be injected (space facing an injection port of injector) by time taken for injecting fuel by opening the injection port of injector (fuel injection time). Therefore, in order to precisely control the injection quantity of fuel, it is necessary to control both of the differential pressure between the fuel pressure and the pressure of the space into which fuel is to be injected and the fuel injection time. However, it is actually difficult to control both of these. Therefore, the injection quantity of fuel is controlled by assuming the differential pressure to be substantially constant and controlling only the fuel injection time.
In the low-speed area of the engine, in order to stabilize idling, it is desirable to precisely control the injection quantity of fuel.
In the case that most of the fuel is injected on the compression stroke after the closure of exhaust port in the low-speed area of the engine as disclosed in the conventional fuel injection control method, most of the fuel is injected in a state in which the differential pressure between the fuel pressure applied to the injector and the pressure of the space into which fuel is to be injected (space in the combustion chamber) changes every moment, so that the injection quantity of fuel cannot be controlled only by controlling the injection time. Therefore, in the case where most of the fuel is injected after the exhaust port has been closed as disclosed in the conventional control method, it is difficult to keep an air-fuel ratio in the idling state of the engine at a desirable value. At the time of idling operation of the engine, since the rotational speed of the engine is low and the inertia is low, if the injection quantity of fuel shifts from the calculated value and thus combustion becomes unstable, the influence of unstable combustion is immediately exerted on the rotation of the engine, so that there arises a problem in that the rotation in idling becomes unstable.
Also, in the conventional fuel injection control method, at the time of high-speed rotation of the engine, the timing of starting fuel injection is delayed considerably from the timing of opening the scavenging port, so that as the rotational speed increases, it becomes difficult to evenly mix the fuel with fresh air flowing into the combustion chamber. Therefore, when the engine rotates at a speed close to the upper limit in the high-speed rotation area, the air-fuel ratio distribution of mixture in the combustion chamber becomes uneven, so that the propagation velocity of flame decreases, which causes a problem that the output torque of the engine decreases.
In the case of an engine used for a vehicle operated under a condition that the running resistance is always substantially constant, for example, an outboard motor, characteristics such that the output torque decreases when the rotational speed is increased in an area close to the upper limit of high-speed area are allowed. However, in the case of an engine used for a vehicle, such as a snow mobile, operated under a condition that the running resistance is very high, it is required to provide so-called peaky characteristics such that at the time when a throttle valve is fully opened, the output torque increases as the rotational speed increases up to an area close to the upper limit of high-speed area. In the conventional control method, at the time of high-speed rotation, the air-fuel ratio distribution in the combustion chamber is uneven, and thus the ignitability of fuel is deteriorated. Therefore, it is difficult to meet such a requirement.
Accordingly, an object of the present invention is to provide a fuel injection control system and a control method for a two-cycle in-cylinder direct injection engine, in which a rotation can be kept steady at the time of idling of the engine.
Another object of the present invention is to provide a fuel injection control system and a control method for a two-cycle in-cylinder direct injection engine, in which not only the rotation can be kept steady at the time of idling of the engine, but also characteristics such that in a high-speed area, an output torque increases with increasing rotational speed of the engine at the time when a throttle valve is fully opened can be obtained.
The present invention is applied to a fuel injection control method of controlling the injection of fuel from an injector of a two-cycle in-cylinder direct injection engine having a combustion chamber, a scavenging port and an exhaust port which are opened in the combustion chamber and are opened and closed by a piston moving vertically in the combustion chamber, the injector installed so that a fuel injection port is opened in the combustion chamber, and a crankshaft connected to the piston. In the present invention, in a low-speed area in which the rotational speed of the engine takes a value lower than a set value, the fuel injection from the injector is controlled so that the fuel injection from the injector is started after the scavenging port of the engine has been closed, and the injection of all fuel is finished during the time when the exhaust port is opened; and in a middle and high-speed area in which the rotational speed of the engine takes a value exceeding the set value, the fuel injection from the injector is controlled so that fuel is injected even for a period of time deviated from the period between the timing of closure of the scavenging port and the timing of closure of the exhaust port to secure the injection time required by the engine.
In this specification, the low-speed area is defined as a rotational speed area before a load to be driven by the engine starts to operate. This low-speed area includes an idling area. Also, in the case where a crankshaft of the engine is connected to a driving shaft of the load via a centrifugal clutch, an area from the upper limit of the idling area to a clutch engagement speed (rotational speed at the time when the clutch engages) is also included in the low-speed area.
Generally, in the low-speed area of the engine, the engine is in a standby state, the load thereof being low, and an opening of a throttle valve is small, so that the quantity of fresh air flowing into the combustion chamber through the scavenging port is throttled, and thus the delivery ratio is kept at a low value. Also, in the low-speed area, since the flow velocity of scavenging air is low, fresh air scarcely blows off, so that the charging efficiency is 80% or more. Therefore, even if all fuel is injected during the time when the exhaust port is opened as described above, the fuel-air mixture scarcely blows off.
Moreover, since the pressure in the combustion chamber is substantially constant in the state in which the exhaust port is opened, the injection quantity of fuel from the injector can be controlled precisely merely by controlling the injection time.
Thereupon, as described above, in the low-speed area of the engine, if all fuel is injected during the time when the exhaust port is opened, the fuel injection quantity in the low-speed area is controlled precisely, so that the combustion of fuel can be operated properly, and thus the idling operation of the engine can be performed steadily. Also, since the combustion in the low-speed area can be operated properly, the yield of HC is decreased, so that the exhaust gas can be purified.
When a two-cycle engine is used as a driving source for a vehicle such as a snow mobile, the crankshaft of the engine is often connected to the driving shaft of the vehicle via the centrifugal clutch. When the present invention is applied to such an engine, the fuel injection is controlled as described below.
That is, in a low-speed area in which the rotational speed of the engine takes a value not higher than a first set value that is lower than a clutch engagement speed which is a rotational speed at the time when the centrifugal clutch engages, the fuel injection from the injector is controlled so that the fuel injection from the injector is started after the scavenging port of the engine has been closed, and the injection of all fuel is finished during the time when the exhaust port is opened. In a middle-speed area in which the rotational speed of the engine takes a value exceeding the first set value and not higher than a second set value that is higher than the clutch engagement speed, the fuel injection from the injector is controlled so that the timing of start of the fuel injection from the injector is delayed from the timing of closure of the scavenging port, and the timing of finish of the fuel injection is delayed from the timing of closure of the exhaust port, by which the quantity of fuel injected after the exhaust port has been closed is increased.
Also, in a high-speed area in which the rotational speed of the engine takes a value exceeding the second set value, the fuel injection from the injector is controlled so that the timing of start of the fuel injection is advanced as the rotational speed increases, and the timing of finish of the fuel injection is delayed from the timing of closure of the exhaust port. The fuel injection from the injector is controlled so that the timing of start of the fuel injection in the high-speed area is finally advanced to the timing of opening of the scavenging port or the timing immediately after the scavenging port opening timing.
If the timing of start of the fuel injection in the high-speed area is advanced to the timing of opening of the scavenging port or the timing immediately after the scavenging port opening timing, fuel is mixed with fresh air properly, and thus the output torque at the time of high-speed rotation can be increased. Therefore, the characteristics such that the output torque increases with increasing rotational speed at the time when the throttle valve is fully opened can be obtained easily.
If at the time of high-speed rotation, the position in which the fuel injection is started is advanced to a position near the position in which the scavenging port is opened as described above, the temperature of fuel-air mixture can be decreased by the latent heat of vaporization of gasoline. Therefore, the occurrence of knocking and detonation can be prevented. Also, the yield of NOx is restrained, so that exhaust gas can be purified at the time of high-speed rotation.
In an engine for driving a vehicle operated under a condition of high running resistance, such as a snow mobile, it is necessary to set the clutch engagement speed, the rotational speed at the time when the centrifugal clutch engages, at a speed in the middle-speed area of, for example, 4000 to 5000 [r/min].
In the middle-speed area of the two-cycle engine including the clutch engagement speed, it is necessary to produce a sufficient output torque to make provisions against a sudden increase in the load at the time when the centrifugal clutch engages. In the middle-speed area of the two-cycle engine, the delivery ratio is about 0.4 to 0.6, and fresh air often blows off, so that it is preferable that the injection start timing is delayed from the timing of closure of the scavenging port to inject much fuel after the exhaust port has been closed.
In the present invention, therefore, in the middle-speed area in which the rotational speed of the engine takes a value exceeding the first set value and not higher than the second set value that is higher than the first set value and a clutch engagement speed, the fuel injection from the injector is started with the timing delayed from the timing of closure of the scavenging port, and much fuel is injected after the exhaust port has been closed.
Also, in the high-speed area, the crank angle necessary for injecting a necessary quantity of fuel increases. Therefore, in the high-speed area of the engine, the timing of start of the fuel injection is advanced as the rotational speed increases, and also the timing of finish of the fuel injection is delayed from the timing of closure of the exhaust port. The timing of start of the fuel injection is finally advanced to the timing of opening of the scavenging port or the timing immediately after the scavenging port opening timing.
In the high-speed area of the engine, the time taken for one rotation of the crankshaft is short, and thus the time that can be used for mixing the injected fuel with fresh air is short. However, if the fuel injection is started at the same time that the scavenging port is opened or immediately after the opening of the scavenging port as described above, fuel is injected continuously during the time when fresh air flows into the combustion chamber through the scavenging port, whereby fuel can be mixed with fresh air properly, and thus the air-fuel ratio distribution in the combustion chamber at the ignition time can be made even. Therefore, if the above-described control is carried out, even in the vicinity of the upper limit of high-speed area, the ignitability of fuel is improved, so that the output torque of the engine can be increased, and also the peaky characteristics such that the output torque increases with increasing rotational speed can be obtained easily.